Value and reference types
Learn the core difference between value types and reference types.
Boxing
Boxing wraps a value type into an object (or interface) box. The value is copied from the stack to the heap, and the reference points to that address.
Example: object box = 42; The int value is placed on the heap and box points to it. Boxing happens automatically and implicitly; you write no extra code.
Boxing is convenient but slow because it allocates heap memory. Whenever possible, use the value type directly for better speed.
Unboxing and performance
Unboxing extracts a boxed object back into a value type. It is the reverse of boxing but requires an explicit cast.
Example: int n = (int)box; The (int) parentheses tell the compiler the type explicitly. Unboxing with the wrong type throws an InvalidCastException.
Boxing and unboxing together are costly; both do heap work. For now, remember that unboxing needs an explicit cast.
Exercise Box and unbox a value
Assign 42 to an object box variable (boxing occurs). Extract the value back with int value = (int)box; (unboxing). Then print value with Console.WriteLine.
Show example solution
object box = 42; int value = (int)box; Console.WriteLine(value);
Example solution:
using System;
class Program {
static void Main() {
object box = 42;
int value = (int)box;
Console.WriteLine(value);
}
}Test yourself
-
Match each conversion to its name
- int to object
- boxing
- object to int
- unboxing
- int to double
- implicit
- string to int
- Parse
-
Which line causes boxing?
- double d = 5;
- object o = 5;
- int n = (int)5.0;
- string s = "5";
Show answer
object o = 5; — Correct! Assigning a value type to object causes boxing.
-
Unbox an object into an int
int n = ____ o;Show answer
int n = (int) o;